The present invention generally relates to the conversion of organic lignocellulosic materials (biomass) into useful fuels (biofuels), and more particularly to a system and process capable of continuous conversion of biomass into synthesis gas (syngas).
Syngas is a gas mixture containing carbon monoxide (CO) and hydrogen gas (H2) produced by the conversion of carbonaceous materials, such as coal, petroleum, and biomass materials. Though having a lower energy density than natural gas, syngas is suitable for use as a fuel source for a variety of applications, including but not limited to gas turbines and automotive internal combustion engines. Syngas can also be used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process to produce a synthetic petroleum substitute.
The use of syngas as a fuel is more efficient than direct combustion of the original biomass because more of the energy contained in the biomass is extracted by the conversion process, known as gasification. Within a typical biomass gasifier, a carbonaceous material is combusted in an atmosphere where the oxygen content is below the stoichiometric limit at which complete combustion can occur. This oxygen-starved combustion of carbonaceous material releases volatiles, in the case of dry feedstock, produces a carbon-rich char, and releases heat. This heat raises the temperature of non-combusted carbonaceous material, causing it to pyrolyze, releasing flammable volatiles such as carbon monoxide (CO), hydrogen (H2) and, depending on the temperatures used, may also produce methane (CH4) and hydrocarbon molecules having a greater number of carbon atoms. This blend of flammable volatiles is termed synthesis gas, or syngas, for short.
In the case of dry feedstock material, it is possible to convert the char into flammable volatiles. One such method is the injection of steam (H2O), which reacts with the char to produce more CO and H2, according to the reactionC+H2O→H2+CO
Consequently, the biomass gasification process employs sub-stoichiometric quantities of oxygen or air to combust a portion of the biomass and through pyrolysis, and the optional injection of steam, produce syngas and heat (energy).
Pyrolysis is an endothermic process, and various heating techniques have been proposed for use in the production of syngas, including but not limited to partial combustion of the biomass products through air injection, direct heat transfer by mixing with a hot gas, indirect heat transfer with exchange surfaces (for example, walls or tubes), and direct heat transfer with circulating solids. Each of these heating techniques has significant technical shortcomings. For example, partial combustion results in poor-quality products, for example, a syngas having an energy content of 150 BTU/ft3 or less, because of the dilution of the fuel gasses by the nitrogen in the injected air and the gaseous products of the combustion. With direct heat transfer, typically with a product gas that is reheated and recycled, a shortcoming is that a very large ratio of recycle gas to feed gas is required to provide sufficient heat with reasonable gas flowrates. For indirect heat transfer, it can be difficult to maintain desired heat transfer rates because the process deposits coatings on the heat transfer surfaces that act as insulating materials. Finally, direct heat transfer with circulating solids is effective but requires complex technology because the circulating solids, which typically transfer heat between a burner and a pyrolysis reactor, involve a moving bed that requires a significant investment in equipment and energy management to be effective in a continuous process.
Various types of gasifier designs are known, including counter-current fixed bed (up-draft) gasifiers, con-current fixed bed (down-draft) gasifiers, fluidized bed gasifiers, and entrained flow gasifiers. The most common type of gasifier used in biomass gasification is believed to be the up-draft design, in which a gasification agent (air, oxygen and/or steam) flows upward through a permeable bed of biomass and counter-currently to the flow of ash and other byproducts of the reaction. These gasifier designs have significant technical shortcomings, particularly if the intent is to produce a syngas having a higher energy content, for example, about 300 BTU/ft3 or more, from cellulosic agricultural residue. Most current available technologies, including up-draft and down-draft fixed beds, fluidized beds, or entrained flow gasifiers, can be either pressurized or non-pressurized (atmospheric) design. As previously noted, the use of air for partial combustion to provide the energy for pyrolysis and gasification introduces a large volume of inert diluting gas (nitrogen), which is the major contributing factor to the production of low BTU syngas. Because biomass is a low-energy content fuel and is dispersed geographically, low-BTU syngas negatively affects the economic payback for the gasifier system. The use of an external heat source and/or pure oxygen would overcome the diluent effect of air to allow for the production of a medium BTU syngas (about 300 BTU/ft3 or more). However, a major problem remains as to how to prevent the ingress of air while allowing the egress of syngas from the feed material ingress and the egress of ash from the spent material outlet.